The invention relates to a filter for absorbing X-rays. More particularly, the invention relates to an X-ray absorbing filter having a controllable matrix for generating a field acting on a liquid located in a housing. In addition, the invention is directed to an X-ray machine having such a filter for absorbing X-rays.
In X-ray examinations, the patient and his/her organs in the region to be examined often exhibit strongly differing absorption characteristics with respect to the X-rays applied. For example, for chest X-rays, the attenuation in the mediastinum (i.e., in the region in front of the pulmonary lobes) is very largely determined by the organs to be found there. In the region of the pulmonary lobe itself, by contrast, the attenuation is small. In order both to obtain an informative picture and, in particular, to spare the patient unnecessary exposure to radiation, it is sensible to set the applied dosage based on the characteristics of the region being examined, in such a way that no more X-rays than necessary are supplied. In other words, a larger dose should be applied in regions of large attenuation than should be applied in regions of low attenuation.
In addition, there are applications in which only a portion of the region to be examined need be recorded with high diagnostic quality, i.e. with little noise. The surrounding parts are important for orientation but not for the actual diagnosis. These surrounding regions could, then, be imaged with a smaller dose of radiation in order thereby to reduce the total dosage applied.
Such a filter is disclosed, for example, in German Laid-Open publication DE-A 44 22 780. This filter has a housing with a controllable electrode matrix that is used to generate an electric field. This field acts on a liquid in contact with the electrode matrix. Ions that absorb X-rays are present in this liquid. These ions move freely, and migrate as dictated by the applied field. Hence, by appropriately forming the field in the region of one or more electrodes, it is possible to accumulate a correspondingly large or small number of ions so as to change the local absorption behavior of the filter. However, the absorption profile that can thereby be achieved over the filter area, i.e. the attenuation pattern caused by ion accumulation, frequently does not provide effects which are satisfactory for specific applications in the fields of diagnosis and therapy. In addition, there is a further disadvantage in that the accumulation requires ions to migrate in the field. It requires a relatively long time in order to achieve a state of equilibrium, in which ion migration ceases. (The equilibrium state is a function of the field.)